Visualizing structures

An introduction to the geometry of geological structures in the field

Goals

• To familiarize you with the 3-dimensionality of geological structures in the field

• Provide you with simple rules to use in geologic mapping of structures

3D spaces to deal with

• Topographic

• Depositional/intrusive

• Deformational

They all intersect

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Shaded

Non-shaded

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Any structures will intersect an irregular topographyTry picturing how will a horizontal layer cut this relief

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Horizontal plane - follows topo contours

• Let us now put the real topography aside and step into the realm of “primary” geologic structures - layering of sedimentary units, intrusions, viewed in an idealized 3D framework

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Complicating the issue a little bit…

Unconformity, what is it?

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Possible scenario: Deposition, erosion, deformation, new deposition

The way this would look in a canyon near Page, AZ

Igneous intrusions

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Structures on block diagrams

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• Let us know try to drape structures onto topography and come up with some simple rules for the way they will plot on maps.

To keep things simple for now, I will deal mainly with planes and not with lines.

So what can a plane be? A bed, a fault, a dike, a sill an unconformity, anything that is planar.

The simplest scenario: a horizontal bed

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Horizontal beds (a’la water line)

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Some shallow tilt of a plane

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Rule of v’s

An example

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• So then what is a geological map?

An overlay of geologic structures (e.g. like the planes shown here on the 2D topographic map). Let’s take a look.

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Tucson Mts.

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Cross-sections

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Remember

• The 3 spaces you deal with: topo, primary structures and deformational (secondary) structures);

• Rule of v’s, overall topography/structure intersections of any planes

• A structure looks very different depending from where you look at it!

A preview of Cottonwood

Be familiar with this lecture and this week’s lab - it will be important for Saturday’s exercise.